Active matrix liquid crystal display device

ABSTRACT

A first insulating thin film having a large dielectric constant such as a silicon nitride film is formed so as to cover a source line and a metal wiring that is in the same layer as the source line. A second insulating film that is high in flatness is formed on the first insulating film. An opening is formed in the second insulating film by etching the second insulating film, to selectively expose the first insulating film. A conductive film to serve as a light-interruptive film is formed on the second insulating film and in the opening, whereby an auxiliary capacitor of the pixel is formed between the conductive film and the metal wiring with the first insulating film serving as a dielectric.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the circuit configuration and layout ofa pixel area of an active matrix display device in which thin-filmtransistors are used and source lines formed above gate lines. Inparticular, the invention relates to the structure of an auxiliarycapacitor.

2. Description of the Related Art

In recent years, techniques of forming thin-film transistors (TFTs) onan inexpensive glass substrate have been made rapid progress. This isbecause of increased demand for the active matrix liquid crystal displaydevice.

In the active matrix liquid crystal display device, thin-filmtransistors are provided for respective ones of hundreds of thousands tomillions of pixels that are arranged in matrix form and the chargeentrance and exit to each pixel is controlled by the switching functionof the thin-film transistor.

A liquid crystal is interposed between each pixel electrode and anopposed electrode, to form a kind of capacitor. Therefore, image displayis realized by controlling the quantity of light passing through theliquid crystal panel by varying the electro-optical characteristic ofthe liquid crystal by controlling the entrance and exit of change to andfrom this capacitor with the thin-film transistor.

The capacitor having the above structure has a problem that since thevoltage held by the capacitor gradually decreases due to currentleakage, it changes the electro-optical characteristic of the liquidcrystal and deteriorates the contrast of image display.

A common measure to solve the above problem is a configuration in whichan additional capacitor called an auxiliary capacitor is provided inparallel with the capacitor including the liquid crystal and chargeequivalent to charge that is lost due leakage etc. is supplied to thecapacitor including the liquid crystal.

FIG. 1 is a circuit diagram of a conventional active matrix liquidcrystal display device. The active matrix display circuit is generallydivided into three parts: a gate driver circuit 2 for driving gate lines(i.e., gate lines, scanning lines) 4, a data driver circuit 1 fordriving source lines (i.e., data lines, source lines or signal lines) 5,and an active matrix circuit 3 that is provided with pixels. The datadriver circuit 1 and the gate driver circuit 2 are generically called aperipheral circuit.

In the active matrix circuit 3, a number of gate lines 4 and sourcelines 5 are provided so as to cross each other and pixel electrodes 7are provided at the respective intersecting points. A switching element(thin-film transistor) 6 is provided to control charge that enters orexits from each pixel electrode 7. Selection is made between thetop-gate thin-film transistor (the gate electrode is formed above theactive layer) and the bottom-gate thin-film transistor (the active layeris formed above the gate electrode) in accordance with the necessarycircuit structure, the manufacturing process, the requiredcharacteristics, and other factors. Further, as described above, toprevent a variation in pixel voltage due to leak current, an auxiliarycapacitor 8 is provided in parallel with each pixel capacitor.

On the other hand, the conductivity of the thin-film transistor isvaried by illumination with light. To prevent this phenomenon, it isnecessary to cover each thin-film transistor with a light-interruptivecoating (black matrix). The light-interruptive coating is formed so asto also cover the portions between the pixels to prevent color orbrightness contamination between the pixels and a display failure due toa disordered electric field at pixel boundaries.

So, the light-interruptive coating assumes a matrix shape and hence iscalled a black matrix (BM). At first, in favor of advantages in amanufacturing process, the black matrix was provided over the substrate(opposed substrate) that opposes the substrate on which the activematrix circuit is formed. However, recently, because of the need forincreasing the area of each pixel (aperture ratio), it is proposed toprovide the black matrix over the substrate on which the active matrixcircuit is formed.

SUMMARY OF THE INVENTION

Various proposals haven made of the structure of the auxiliarycapacitor. However, it is difficult to obtain a large capacitance whilemaintaining the area of the open portion (light-transmissive portion) ofeach pixel.

The present invention has been made in view of the above circumstancesin the art, and an object of the invention is therefore to provide astructure of an auxiliary capacitor which can provide a largecapacitance while maintaining the area of the open portion(light-transmissive portion) of each pixel.

According to one aspect of the invention there is provided an activematrix liquid crystal display device comprising a thin-film transistorhaving a source region to which a pixel electrode is electricallyconnected; a drain electrode connected to a drain region of thethin-film transistor and formed in the same layer as a source line, thedrain electrode having a pattern that covers 50% or more of an activelayer of the thin-film transistor; and an auxiliary capacitor formed byusing the drain electrode.

With the above configuration, the aperture ratio of the pixel can beincreased because the auxiliary capacitor is formed above the thin-filmtransistor.

Another aspect of the invention attains the above object by forming aconductive light-interruptive film over the active-matrix-sidesubstrate, keeping it at a constant potential, and using it as oneelectrode of the auxiliary capacitor. Since originally thelight-interruptive film does not transmit light, the aperture ratio doesnot decrease even if it is used as one electrode of the auxiliarycapacitor.

The active matrix liquid crystal display device of the inventioncomprises:

(1) a thin-film transistor;

(2) a gate line and a source line formed above the gate line;

(3) a conductive film serving as a light-interruptive film and kept at aconstant potential;

(4) a metal wiring connected to a drain region of the thin-filmtransistor and made of the same layer as the source line; and

(5) an interlayer insulating film formed between the source line and theconductive film, and comprising at least two insulating layers.

In the invention, the thin-film transistor may be of either the top gatetype or the bottom gate type as long as the above conditions aresatisfied. This is, since the main improvements of the invention relateto the structure above the source line, the structure below the sourceline (i.e., the positional relationship between the gate line and theactive layer) is irrelevant. Also, the interlayer insulating layer mayconsist of three or more layers.

According to another aspect of the invention, in the aboveconfiguration, an auxiliary capacitor having the metal wiring and theconductive film (light-interruptive film) as electrodes and at least thelower insulating layer of the interlayer insulating film as a dielectricis formed in a region where the upper insulating layer of the interlayerinsulating film is removed by etching. The dielectric may consist of twoor more insulating layers.

According to a further aspect of the invention, in the aboveconfiguration, the conductive film (light-interruptive film) overlapswith the metal wiring and has a portion that is in contact with thelower insulating layer.

In the two aspects of the invention just mentioned above, it iseffective to employ, as the main component of the lower insulatinglayer, silicon nitride that is produced stably in semiconductorprocesses and has a large relative dielectric constant. In this case,the dielectric of the auxiliary capacitor may be composed of only asilicon nitride layer or may have a multi-layer structure of a siliconnitride film and some other coating (for instance, a silicon oxidefilm).

In this case, the dielectric is made thinner and the use of siliconnitride having a large dielectric constant realizes a large capacitance.In the invention, the thickness of the silicon nitride layer is set at1,000 Å or less, preferably 500 Å or less.

In this configuration, since the silicon nitride film covers the activematrix circuit from above the source lines, the barrier function ofsilicon nitride resulting from its high moisture resistance, highresistance to ions, etc. can be utilized effectively.

In the invention, it is effective to form the upper insulating layer byusing an organic resin, which is easy to be planarized (for instance,polyimide, polyamide, polyimideamide, epoxy, or acrylic). In this case,since the organic resin is insufficient in barrier function (themoisture resistance, the resistance to ions, etc. are low), it isdesirable that the lower insulating layer be made of a materialexhibiting a superior barrier function such as silicon nitride, aluminumoxide, or aluminum nitride.

In the invention, it is effective to provide the metal wiring in aregion of each pixel where disclination (alignment disorder of liquidcrystal molecules due to irregularity or a lateral electric field) isprone to occur. Among various kinds of disclination, disclination due todust or the like can be eliminated by cleaning of a manufacturingprocess. However, disclination caused by irregularity in the devicestructure (for instance, irregularity in the vicinity of a pixelelectrode contact) or a lateral electric field cannot be eliminatedthoroughly. It is not proper to use, for display, a pixel region wheredisclination occurs. Conventionally, such a region is covered with alight-interruptive film so as not to serve for display. In contrast, inthe invention, the auxiliary capacitor can be provided in such a region,whereby the available area of each pixel can be utilized efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a common active matrix circuit;

FIGS. 2A and 2B are top views showing a manufacturing process of anactive matrix circuit according to a first embodiment of the presentinvention;

FIGS. 3A-3E are sectional views showing the manufacturing process of anactive matrix circuit according to the first embodiment;

FIGS. 4A and 4B are top views showing a manufacturing process of anactive matrix circuit according to a second embodiment of the invention;

FIG. 5 shows how disclination occurs;

FIGS. 6A and 6B are top views showing a manufacturing n process of anactive matrix circuit according to a third embodiment of the invention;

FIGS. 7A and 7B are a schematic top view and a circuit diagram of athin-film transistor according to fourth embodiment of the invention;

FIGS. 8A and 8B are top views showing a manufacturing process of anactive matrix circuit according to the fourth embodiment;

FIGS. 9A and 9B are top views showing a manufacturing process of anactive matrix circuit according to a fifth embodiment of the invention;

FIG. 10 is a top view of an active matrix circuit according to amodification of the fifth embodiment;

FIGS. 11A-11D are sectional views showing a manufacturing process of theactive matrix circuit according to the fifth embodiment; and

FIGS. 12-14 are top views showing the configuration of an active matrixcircuit according to a sixth embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIGS. 2A-2B and FIGS. 3A-3E are top views and sectional views,respectively, showing a manufacturing process according to thisembodiment. The reference numerals used in FIGS. 2A-2B and

FIGS. 3A-3E correspond to each other. Numerical values of the filmthickness etc. used in the following embodiments are just examples andare not necessarily optimum ones, and a party to practice the inventionis completely allowed to change those values when necessary.

First, a 500-Å-thick amorphous silicon film is formed over a glasssubstrate 11 by plasma CVD or low-pressure CVD. It is preferable to forma 3,000-Å-thick silicon oxide film as an underlayer film on the glasssubstrate 11 by sputtering or plasma CVD. The underlayer film may beomitted in a case of using a quartz glass substrate.

Then, an active layer 12 of a thin-film transistor is obtained byconverting the amorphous silicon film into a crystalline silicon film bya known annealing technique such as heating or laser light illuminationand etching the crystalline silicon film.

Then, a 1,000-Å-thick silicon oxide film 13 as a gate insulating film isformed by plasma CVD, low-pressure CVD, or sputtering. A gate line (gateelectrode) 14 is then obtained by forming and a 5,000-Å-thickpolysilicon film containing phosphorus by low-pressure CVD and etchingit (see FIG. 3A).

Subsequently, a source 15 and a drain 16 are formed by implanting, intothe active layer 12, ions of phosphorus that is an impurity forimparting n-type conductivity at a dose of 5×10¹⁴ to 5×10¹⁵ atoms/cm².The source 15 and the drain 16 are given n-type conductivity. After theimplantation of impurity ions, the impurity-ion-implanted regions areactivated by performing a heat treatment or illumination with laserlight or strong light.

Then, after a 5,000-Å-thick silicon oxide interlayer insulating film 17is formed, contact holes reaching the source 15 and the drain 16 areformed by etching both the interlayer insulating film 17 and the gateinsulating film 13. Then, a source line 18 and a metal wiring (auxiliarycapacitor electrode) 19 are formed by a known metal wiring formingtechnique (see FIG. 3B).

FIG. 2A is a top view showing a circuit that has been formed by theabove steps.

Thereafter, a silicon nitride film 20 is formed at a thickness of250-1,000 Å A this embodiment, 500 Å) by a plasma CVD method that usessilane and ammonia, silane and N₂O, or silane, ammonia, and N₂O.Alternatively, the silicon nitride film 20 may be formed by usingdichlorosilane and ammonia. As a further alternative, it may be formedby low-pressure CVD, photo CVD, or other proper methods.

Subsequently, a polyimide layer 21 is formed by spin coating at athickness of at least 8,000 Å, preferably 1.5 μm. The surface of thepolyimide layer 21 is planarized. An interlayer insulating filmconsisting of the silicon nitride layer 20 and the polyimide layer 21 isthus formed. Then, an opening 22 for an auxiliary capacitor is formed byetching the polyimide layer 21 (see FIG. 3C).

Depending on the etchant used, the silicon nitride layer 20 may also beetched in the step of etching the polyimide layer. 21. Therefore, toprotect the silicon nitride film 20, a silicon oxide film having athickness of 50-500 Å, for instance, 200 Å, may be formed between thesilicon nitride layer 20 and the polyimide layer 21.

Then, a 1,000-Å-thick titanium film is formed by sputtering. It goeswithout saying that some other metal film such as a chromium film or analuminum film may be formed, and that other proper film forming methodsmay be used. A black matrix 23 is formed by etching the titanium film soas to cover the opening 22 for an auxiliary capacitor (see FIG. 3D).

FIG. 2B is a top view showing the opening 22 for an auxiliary capacitorand the black matrix 23 that have been formed by the above steps. Anauxiliary capacitor is formed in the region where the opening 22 and theblack matrix 23 overlap each other. A contact hole for a pixel electrodewill be formed later in a contact region 31 where the metal wiring 19and the black matrix 23 do not overlap.

Then, after a 5,000-Å-thick polyimide film 24 is formed as an interlayerinsulating film, a contact hole reaching the metal electrode 19 isformed by etching the polyimide films 21 and 24 in the contact region31. A pixel electrode 25 is then formed by forming a 1,000-Å-thick ITO(indium tin oxide) film by sputtering and etching it (see FIG. 3E).

An active matrix circuit is thus completed. An insulating film made ofpolyimide, like the one used in this embodiment, can easily beplanarized and hence is very advantageous. In this embodiment, theauxiliary capacitor is formed in the region 22 where the black matrix 23and the metal wiring 19 are in close proximity to each other. Thesilicon nitride layer 17 serves as a dielectric.

Embodiment 2

FIGS. 4A and 4B are top views showing a manufacturing processingaccording to this embodiment. The manufacturing process itself of thisembodiment is almost the same as that of the first embodiment. Thereference numerals commonly used in the first and second embodimentsrepresent the same or equivalent parts. This embodiment is differentfrom the first embodiment in circuit layout; that is, each pixel isformed efficiently (i.e., the effective aperture ratio is increased) byforming the auxiliary capacitor in a region where disclination is proneto occur.

FIG. 5 shows a pixel having the same circuit layout as the pixelaccording to the first embodiment. As shown in FIG. 5, disclination isprone to occur in a top-right region 30 of the pixel in a display devicein which a pixel electrode contact 31 is provided at a top-rightposition of the pixel, rubbing is performed in the top-right tobottom-left direction (not bottom-left to top-right direction), and thesource-line-inverted driving is performed. (The source-line-inverteddriving is a driving method in which signals of opposite polarities areapplied to adjacent source lines, and includes the dot-inverteddriving). Since the region 30 is not suitable for use for display, it isdesired to cover it with a black matrix.

In view of the above, in this embodiment, a metal wiring 19 is providedin a right-hand region of the pixel as shown in FIG. 4A rather than inthe top portion as in the case of the first embodiment.

Further, an opening 22 is formed in the metal wiring 19 and is coveredwith a black matrix 23. It is effective to form a contact for a pixelelectrode in a bottom-right region 31 as shown in FIG. 4B.

In this manner, the auxiliary capacitor is formed in the region wheredisclination is prone to occur. In this embodiment, the auxiliarycapacitor that is provided in the top portion of the pixel in thecircuit of the first embodiment is moved to the right-hand region andhence the area of the opening remains the same in terms of the circuitdesigning. However, the effective opening area can be increased byoverlapping the disclination and the auxiliary capacitor (or BM) witheach other.

Embodiment 3

FIGS. 6A and 6B are top views showing a manufacturing processingaccording to this embodiment. The manufacturing process itself of thisembodiment is almost the same as that of the first embodiment. Thereference numerals commonly used in the first and third embodimentsrepresent the same or equivalent parts. Although the layout relating tothe auxiliary capacitor in this embodiment is substantially the same asin the second embodiment, in this embodiment it is intended to utilizethe available area of each pixel more efficiently by changing the layoutrelating to the active layer of the thin-film transistor.

In this embodiment, rubbing is performed in the bottom-left to top-rightdirection, in which case disclination is prone to occur in a bottom-leftregion. While in the second embodiment the auxiliary capacitor isprovided in such a region where disclination is prone to occur, in thisembodiment part of the active layer of the thin-film transistor of thenext row is additionally formed in this region. That is, as shown inFIG. 6A, a metal wiring 19 is provided in a left-hand region of thepixel and an active layer 12 is formed so as to cross a gate line 14,that is straightened (i.e., the branch portion of the gate line isremoved).

Further, an opening 22 is formed in a metal wiring 19 and is coveredwith a black matrix 23 (see FIG. 6B).

In this manner, the auxiliary capacitor and part of the thin-filmtransistor are formed in the region where disclination is prone tooccur. According to this embodiment, the available area of each pixelcan be utilized more efficiently by a degree corresponding to theremoval of the branch portion of the gate line.

Embodiment 4

FIGS. 7A and 7B are a top view and a circuit diagram of a thin-filmtransistor according to this embodiment, and FIGS. 8A and 8B are topviews showing a manufacturing process according to this embodiment. Themanufacturing process itself of this embodiment is almost the same asthat of the first embodiment. The reference numerals commonly used inthe first and fourth embodiments represent the same or equivalent parts,and the reference numerals used in FIGS. 7A-7B and 8A-8B correspond toeach other. Although the layout relating to the auxiliary capacitor inthis embodiment is substantially the same as in the second embodiment,in this embodiment it is intended to utilize the available area of eachpixel more efficiently by improving the characteristics of the thin-filmtransistor by changing the layout relating to the active layer of thethin-film transistor and the gate electrode.

In this embodiment, rubbing is performed in the bottom-left to top-rightdirection as in the case of the third embodiment and hence disclinationis prone to occur in a bottom-left region. While in the secondembodiment the auxiliary capacitor is provided in such a region and inthe third embodiment the auxiliary capacitor and part of the activelayer of a single-gate thin-film transistor are formed in this region,in this embodiment the active layer of a triple-gate thin-filmtransistor and the gate electrode are provided in this region as well asthe auxiliary capacitor.

First, a triple-gate thin-film transistor used in this embodiment willbe outlined with reference to FIG. 7A. This thin-film transistor isconfigured in such a manner that a gate line 14 is formed with a branchportion 29 and an active layer 12 overlaps with the gate line 14 and itsbranch portion 29 as shown in FIG. 7A. Transistors are formed atrespective overlap portions 26-28.

That is as shown in FIG. 7B, three thin-film transistors are formed inseries between a source line 18 and a metal wiring 19.

It is known that it is particularly effective to use this type ofmultiple transistor as a switching transistor of an active matrix (referto Japanese Examined Patent Publication No. Hei. 5-44195).

Although the thin-film transistor having the above structure occupies abottom-left region of the pixel of the next row, this does not reducethe aperture ratio as in the case of the second and third embodimentsbecause this region is a region where disclination is prone to occur.That is, as shown in FIG. 8A, the gate line 14 is provided with thebranch portion 29 and the active layer 12 is formed so as to cross thegate line 14 and its branch portion 29 three times in total. Further, ametal wiring 19 is formed in a left-hand region of the pixel as shown inFIG. 8A.

Further, an opening 22 is formed in the metal wiring 29 and is coveredwith a black matrix 23 (see FIG. 8B).

In this manner, the auxiliary capacitor and part of the thin-filmtransistor are formed in the region where disclination is prone tooccur. This embodiment is disadvantageous as compared to the thirdembodiment in that the gate line needs the branch portion as in the caseof the circuit of the second embodiment, by virtue of the use of thetriple-gate thin-film transistor the auxiliary capacitance may be farsmaller than in the third embodiment. Therefore, on balance, thisembodiment is superior to the third embodiment in terms ofcharacteristics.

Embodiment 5

FIGS. 9A-9B and FIGS. 11A-11D are top views and sectional views,respectively, showing a manufacturing process according to thisembodiment. The reference numerals used in FIGS. 9A-9B and FIGS. 11A-11Dcorrespond to each other, and the reference numerals commonly used inthis embodiment and the above embodiments represent the same orequivalent parts. The layout relating to the auxiliary capacitor in thisembodiment is different from that in the pixel circuit having thelaminate structure of the first embodiment.

As in the case of the first embodiment, a 500-Å-thick amorphous siliconfilm is formed, by plasma CVD or low-pressure CVD, on a glass substrate11 on which a proper underlayer film is formed. Then, an active layer 12of a thin-film transistor is obtained by converting the amorphoussilicon film into a crystalline silicon film by a known annealingtechnique and etching the crystalline silicon film.

Then, a 1,000-Å-thick silicon oxide film 13 is formed as a gateinsulating film. A gate line (gate electrode) 14 is then obtained byforming a 5,000-Å-thick polysilicon film containing phosphorus bylow-pressure CVD and etching it (see FIG. 11A).

Subsequently, a source 15 and a drain 16 are formed by implanting, intothe active layer 12, ions of phosphorus that is an impurity forimparting n-type conductivity at a dose of 5×10¹⁴ to 5×10¹⁵ atoms/cm².Annealing is performed after the implantation of impurity ions.

Thereafter, a 2-μm-thick silicon oxide interlayer insulating film 17 isformed by a known insulating layer forming technique, and its surface isplanarized by a known planarization technique (for instance, chemicalmechanical polishing (CMP)). Then, contact holes reaching the source 15and the drain 16 are formed by etching the interlayer insulating film 17and the gate insulating film 13. Then, a source line 18 and a metalwiring (auxiliary capacitor electrode) 19 are formed by a known metalwiring forming technique. At this time, the metal wiring 19 is formed soas to cover the gate line 14 (see FIG. 11B).

FIG. 9A is a top view showing a circuit that has been formed by theabove steps. This embodiment has a feature that the metal wiring 19 toserve as an electrode of the auxiliary capacitor partially covers thegate line 14. Being light-interruptive, both of the gate line 14 and themetal wiring 19 are factors of reducing the area of the pixel regionthat is usable for display. In the first embodiment, they are arrangedso as not to overlap with each other and hence the area of the pixelregion that is usable for display is reduced accordingly. Thisembodiment enables a larger area of the pixel to be used for displaybecause of the structure that the gate line 14 and the metal wiring 19overlap with each other.

Where the gate line 14 for driving the pixel electrode concerned and themetal wiring 19 that is connected to the pixel electrode overlap witheach other as shown in FIG. 9A, it is preferable to weaken thecapacitance coupling between the gate line 14 and the metal wiring 19.In this embodiment, this is done by making the interlayer insulatingfilm 17 sufficiently thick. Alternatively, the metal wiring 19 may beoverlapped with the gate line 14 of the next row.

Thereafter, a silicon nitride film 20 is formed at a thickness of250-1,000 Å (in this embodiment, 500 Å). A 200-Å-thick silicon oxidefilm (not shown) is then deposited. Subsequently, a polyimide layer 21is formed by spin coating at a thickness of at least 8,000 Å or more,preferably 1.5 μm. The surface of the polyimide layer 21 is planarized.An interlayer insulating film consisting of the silicon nitride layer 20and the polyimide layer 21 is thus formed. Then, an opening 22 for anauxiliary capacitor is formed by etching the polyimide layer 21 (seeFIG. 11C).

Then, a 1,000-Å-thick titanium film is formed by sputtering. A blackmatrix 23 is formed so as to cover the opening 22 for an auxiliarycapacitor by etching the titanium film.

FIG. 9B is a top view showing the opening 22 for an auxiliary capacitorand the black matrix 23 that have been formed by the above steps. Anauxiliary capacitor is formed in the region where the opening 22 and theblack matrix 23 overlap. To increase the area of the opening portion, itis preferable to form the opening 22 for an auxiliary capacitor so as tooverlap with the gate line 14. To form a contact hole for a pixelelectrode, a region 31 where the metal wiring 19 and the black matrix 23do not overlap is provided.

Then, after a 5,000-Å-thick polyimide film 24 is formed as an interlayerinsulating film, a contact hole reaching the metal electrode 19 isformed by etching the portions of the polyimide films 21 and 24 in theregion 31. A pixel electrode 25 is then formed by forming a1,000-Å-thick ITO (indium tin oxide) film by sputtering and etching it(see FIG. 11D).

An active matrix circuit is thus completed. Although this embodiment isdirected to the case of using the single-gate TFT, a similar pixelcircuit may be obtained by using a multi-gate TFT, in which case thesame advantages are obtained.

Embodiment 6

This embodiment will be described below with reference to FIGS. 12-15.FIG. 12 shows active layers 105-108 that are formed in the lowest layer,i.e., on a glass substrate, a quartz substrate, or some other insulatingsurface.

A gate insulating film (not shown) is formed on the active layers105-108. Gate lines 101 and 102 are formed on the gate insulating film.

The portions of each of the active layers 105-108 where the gate line101 or 102 crosses the active layer become channel forming regions.

An interlayer insulating film (not shown) is formed on the gate lines101 and 102, and source lines 103 and 104 are formed on the interlayerinsulating film.

For example, the source line 104 is connected to the source region thatis formed in the active layer 106 via a contact 109.

Drain electrodes 109-112 are formed by using the same material as thesource lines 103 and 104 are done (i.e., by patterning the same film asthe source lines 103 and 104 are done).

The drain electrodes 109-112 will be used to form capacitors andconstitute parts of a black matrix.

An extension 113 of the drain electrode 112 is a pattern to increase thecapacitance.

Each of the drain electrodes 109-112 is shaped so as to cover half ormore of the active layer. With this structure, a desired auxiliarycapacitance can be obtained without a large reduction in aperture ratio.

FIG. 13 shows a state that after the state of FIG. 12 a silicon nitridefilm (not shown) has been formed and capacitor lines 1113 and 1114 havebeen formed thereon. The silicon nitride film serves as a dielectric ofeach auxiliary capacitor.

FIG. 14 shows a state that after the state of FIG. 13 an interlayerinsulating film has been formed on the capacitor lines 1113 and 1114 andITO pixel electrodes 115-123 have been formed on the interlayerinsulating film.

In the configuration of this embodiment, the auxiliary capacitor isformed so as to cover the TFT and hence the aperture ratio of the pixelcan be maximized.

Further, a large capacitance can be obtained by forming the capacitorbetween the capacitor line and the drain electrode that is formedbetween the drain region and the pixel electrode at the same time as thesource line. This is because this configuration allows the dielectricfilm (in this embodiment, the silicon nitride film) that constitutes theauxiliary capacitor to be made thinner.

As described above, according to the invention, the conductive film usedas a black matrix is used as an electrode and the auxiliary capacitor isformed between this conductive film and the metal wiring that is in thesame layer as the source line.

With this configuration, the aperture ratio of the pixel can beincreased because the top portion of the TFT is used to form acapacitor.

Although the embodiments are directed to the case of using the top-gateTFT, it is apparent that the invention can similarly be applied to thecase of using the bottom-gate TFT because the invention is animprovement in the structure above the source line.

Having the above advantages, the invention is useful from the industrialviewpoint.

1. A display device comprising: a thin film transistor over a substrate;a first insulating film over the thin film transistor, the firstinsulating film having an opening; a conductive film over the firstinsulating film, the conductive film being in contact with a sidesurface of the first insulating film in the opening; a second insulatingfilm over the conductive film; and a pixel electrode over the secondinsulating film, the pixel electrode being connected to one of a sourceand a drain of the thin film transistor, wherein the pixel electrode andthe conductive film overlaps with each other, and wherein the conductivefilm is kept at a constant potential.
 2. The display device according toclaim 1, further comprising a third insulating film over the thin filmtransistor and under the first insulating film, wherein the conductivefilm is in contact with the third insulating film in the opening.
 3. Thedisplay device according to claim 2, further comprising a wiringconnected to the one of the source and the drain of the thin filmtransistor, wherein a capacitor is formed by the conductive film, thethird insulating film and the wiring in the opening.
 4. The displaydevice according to claim 1, wherein the second insulating filmcomprises polyimide.
 5. The display device according to claim 1, whereinthe thin film transistor is a top-gate thin film transistor.
 6. Thedisplay device according to claim 1, wherein the thin film transistor isa bottom-gate thin film transistor.
 7. The display device according toclaim 1, wherein the conductive film is a light-interruptive film. 8.The display device according to claim 1, wherein the opening of thefirst insulating film is provided within a pixel.
 9. A display devicecomprising: a thin film transistor over a substrate; a first insulatingfilm over the thin film transistor, the first insulating film having anopening; a conductive film over the first insulating film, theconductive film being in contact with a side surface of the firstinsulating film in the opening; a second insulating film over theconductive film; and a pixel electrode over the second insulating film,the pixel electrode being connected to one of a source and a drain ofthe thin film transistor, wherein the conductive film overlaps a channelformation region of the thin film transistor, wherein the pixelelectrode and the conductive film overlaps with each other, and whereinthe conductive film is kept at a constant potential.
 10. The displaydevice according to claim 9, further comprising a third insulating filmover the thin film transistor and under the first insulating film,wherein the conductive film is in contact with the third insulating filmin the opening.
 11. The display device according to claim 10, furthercomprising a wiring connected to the one of the source and the drain ofthe thin film transistor, wherein a capacitor is formed by theconductive film, the third insulating film and the wiring in theopening.
 12. The display device according to claim 9, wherein the secondinsulating film comprises polyimide.
 13. The display device according toclaim 9, wherein the thin film transistor is a top-gate thin filmtransistor.
 14. The display device according to claim 9, wherein thethin film transistor is a bottom-gate thin film transistor.
 15. Thedisplay device according to claim 9, wherein the conductive film is alight-interruptive film.
 16. The display device according to claim 9,wherein the conductive film overlaps entirety of the channel formationregion of the thin film transistor.
 17. The display device according toclaim 9, wherein the opening of the first insulating film is providedwithin a pixel.